JP2001050923A - Hydrogen-gas detecting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hydrogen-gas detecting element whose sensitivity tends not to deteriorate, even when it is exposed to hydrogen at a high concentration. SOLUTION: In this hydrogen-gas detecting element, a sensitive layer 2 which covers a noble metal wire 1 and which is formed of a semiconductor composed mainly of indium oxide is formed, 0.4 to 10 mol% of the oxide of at least one kind of a metal selected from among lanthanide metals is added to the sensitive layer 2, and a dense silica this film 3 is formed on the surface of the sensitive layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen gas sensing element, and more particularly to a semiconductor type gas sensing element having a sensitive layer formed of a semiconductor containing indium oxide semiconductor as a main component and covering a noble metal wire. About. Such a hydrogen gas detecting element is mainly used for monitoring gas leakage and the like in a chemical factory, a semiconductor manufacturing factory, a hydrogen fuel cell for electric vehicles, an engine system, and the like that use hydrogen gas as a reducing agent, a carrier gas, and a fuel. Used in.

[0002]

2. Description of the Related Art Heretofore, as this type of hydrogen gas detecting element, a gas detecting element provided with a sensitive layer formed of a semiconductor mainly composed of a tin oxide semiconductor over a noble metal wire is known. Attempts have been made to develop a hydrogen gas detecting element in which at least one lanthanum-based metal oxide is added to the sensitive layer to improve the selectivity of hydrogen gas at low concentrations.

[0003]

It has been found that the above-mentioned conventional hydrogen gas detecting element detects high-concentration hydrogen gas of 100 ppm or less with high sensitivity, and is used for initial detection of hydrogen gas leakage. It is expected to be effective. This is because hydrogen gas has a property that it has a small molecular radius and easily leaks from an extremely small pinhole, etc. In addition, the lower limit of hydrogen gas explosion is as low as 4% (Vol), and the explosive gas concentration region is low. Because of its large size, early warning of gas explosion is required. However, such a hydrogen gas detecting element has a drawback that once it is exposed to a high concentration of hydrogen gas, the sensitivity is deteriorated.
Specifically, the above-described hydrogen gas detection element is an excellent sensor that works with high sensitivity and high selectivity at a hydrogen gas concentration of 100 ppm or less, but the sensitivity decreases when exposed to hydrogen gas of 500 ppm or more. An experimental result of deterioration has been obtained.

[0004] Accordingly, an object of the present invention is to provide a hydrogen gas detecting element which has high humidity stability and is hardly deteriorated in sensitivity even when exposed to a high concentration of hydrogen in view of the above-mentioned drawbacks.

[0005]

In order to achieve this object, a feature of the hydrogen gas detecting element of the present invention is to provide a sensitive layer formed of a semiconductor containing indium oxide as a main component so as to cover a noble metal wire. A gas sensing element, wherein at least 0.4 to 10 mol% of an oxide of at least one metal selected from lanthanide metals is added to the sensitive layer, and a dense silica thin film, or
The point is that a silica thin film having a hydrogen selective permeability or a silica thin film formed by a chemical vapor deposition process is formed. Further, the sensitive layer contains 0.4 to 10 mol of an oxide of at least one metal selected from lanthanide metals in the sensitive layer.
% Of indium oxide, wherein the silica thin film is passed through the noble metal wire in hexamethyldisiloxane gas at 350 ° C. to 550 ° C. for 25 minutes to
It is desirable that the substrate be formed by a chemical vapor deposition process under a condition of 25 hours.

[Operation and effect] A technique of depositing a silica thin film on a sensitive layer mainly composed of a tin oxide semiconductor or the like is known (see JP-A-56-168542), and such a thin film is formed densely. Accordingly, a technique for restricting the sensitive layer from contacting with a gas other than hydrogen gas and increasing the selectivity of hydrogen gas has been put into practical use (Japanese Patent Publication No. 61-3142).
No. 2). However, with respect to a sensitive layer having a specific composition, although a technique for forming a dense thin film is known, in order to form a dense thin film, the properties of a base material on which the thin film is to be formed, etc. The properties may be greatly affected, and various studies are required depending on the type of the sensitive layer. It is difficult to obtain a silica thin film that selectively transmits only hydrogen gas. Is also hard to predict.

[0007] However, the present inventors have paid attention to the function of the silica thin film, and are thought to exhibit durability against exposure to hydrogen gas. For example, Ce (cerium), Pr (praseodymium), Tb ( As a result of diligent study using a gas sensing element having a sensitive layer containing indium oxide as a main component to which 0.4 to 10 at% of an oxide of at least one metal selected from lanthanide metals such as terbium) is added. When a dense silica thin film is formed on a hydrogen-selective sensitive layer mainly composed of an indium oxide semiconductor, a new finding is obtained that the gas sensing element exhibits the above-described hydrogen gas selectivity.

That is, such a hydrogen gas detecting element is
The high-performance sensing element further improves the hydrogen gas selectivity of the sensitive layer and also has improved durability against high-concentration hydrogen gas. Here, when the silica thin film is formed so as to selectively transmit hydrogen gas, the selectivity at the time of detecting hydrogen gas can be increased, which is effective and formed by a chemical vapor deposition process. Then, it is possible to further effectively prevent deterioration due to the high concentration gas. Further, the silica thin film is formed by passing a current through the noble metal wire in a hexamethyldisiloxane gas and performing a chemical vapor deposition process at 350 ° C. to 550 ° C. for 25 minutes to 25 hours. If so, any of the characteristics can be simultaneously improved, so that a gas sensing element having extremely excellent stability can be provided.

It is considered that the above-mentioned effects are obtained for the following reasons. When a silica thin film is formed on the sensitive layer, as shown in FIG. 32 (schematic diagram), silica crystals close the pores of the sensitive layer, which is usually porous, corresponding to the properties of the sintered surface of indium oxide. Or a thin film having an extremely fine porous structure is formed on the outer surface of the sensitive layer to form a dense layer having the function of a molecular sieve. A gas such as ethanol having a large molecular size cannot pass through the dense layer of silica, and only hydrogen gas mainly passes through the silica thin film to reach the sensitive layer.
The hydrogen gas that reaches the sensitive layer contacts the indium oxide,
It reacts with the surface adsorbed oxygen to generate water molecules and free electrons. The free electrons generated at this time are measured as a hydrogen detection output. However, since the generated water molecules pass through the sensitive layer and are released to the outside, the water side of the silica thin film is sensitive to the sensitive layer. Then, the adsorbed oxygen becomes insufficient. Then, on the sensitive layer side of the silica thin film, since the oxygen gas permeation rate of the silica thin film is slower than that of the hydrogen gas, the gas tends to be delayed, and the gaseous oxygen concentration also decreases. Gaseous oxygen is less likely to adversely affect the reaction,
Even with a small amount of hydrogen gas, free electrons are surely generated, so that reliable gas detection can be performed. In addition, the oxidation number of indium is known to be stable only when it is trivalent, and it is considered that it is unlikely to exist stably in a reduced state. Since the oxidation number easily returns to the original value, it is considered that the behavior is stable. Furthermore, lattice oxygen of indium oxide is known to have high ionicity, and surface adsorbed oxygen is thermally stable and has low oxidizing activity, so it is considered to be advantageous for strong reducing power of hydrogen. .

As a result, while having high sensitivity to hydrogen gas, there is little deterioration even when exposed to high concentration hydrogen gas of 1000 ppm or more, and the humidity dependency is small. Also provided a hydrogen gas detection element that is hardly poisoned. This has made it possible to detect gas with high reliability over a wide concentration range.

Further, in the gas sensing element thus obtained, the output increase rate with respect to the increase in the hydrogen gas concentration is maintained high over a wide gas concentration region (the linearity in the graph of hydrogen gas concentration-output is high). ), Which is useful for knowing an accurate gas concentration in a wide concentration range.

The above-mentioned lanthanide metal is expected to have almost the same effect. It has been experimentally confirmed that a similar effect can be obtained with a kind of metal.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. Commercially available indium hydroxide (In)
By firing the fine powder of (OH) ₃ ) using an electric furnace, an indium oxide powder can be obtained. This indium oxide is further pulverized to form a fine powder, formed into a paste using a dispersion medium such as 1.3-butanediol, applied in a spherical shape covering the noble metal wire 1, and after drying, a current is passed through the noble metal wire 1. Then, sintering was performed in the air to obtain a hot-wire type semiconductor gas detection element including only the sensitive layer 2. The hot-wire semiconductor gas sensing element is impregnated with a solution of a salt of at least one metal selected from lanthanide metals, dried and calcined to cause the sensitive layer 2 to carry various metals in the form of oxides.
The hot-wire semiconductor gas detection element thus produced is heated in an environment of, for example, a saturated vapor pressure of hexamethyldisiloxane (HMDS) (about 9 vol% at 35 ° C.).
The heating is adjusted so that current flows through the noble metal wire 1 to generate Joule heat so that the entire sensitive layer 2 becomes higher than the decomposition temperature of hexamethyldisiloxane. Then
Hexamethyldisiloxane in the atmosphere is thermally decomposed to form a dense silica thin film 3 on the surface of the sensitive layer 2, which is used as a hydrogen gas detecting element (see FIG. 1).

This hydrogen gas detecting element was incorporated in a bridge circuit shown in FIG. 2 and used as a gas detecting device. At this time, the sensor output (output) is obtained by the following equation.

V = -E ｛rs / (rs + r0) -r1 /
(R1 + r2)｝ Here, each variable is as follows. V: sensor output E: bridge voltage rs: resistance of the hot-wire semiconductor gas detection element Rs r0: resistance of the fixed resistance R0 r1: resistance of the fixed resistance R1 r2: resistance of the fixed resistance R2

The sensitivity was determined as the difference between the output in the air coexisting with the detected gas and the output in the clean air. When the sensitivity is expressed as the relative sensitivity, the ratio is defined as a ratio where the sensitivity output under a specific condition is set to 1 and the sensitivity under another condition is indicated.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. Example 1 Commercially available indium hydroxide (In (OH)
₃ ) Indium oxide powder is obtained by baking fine powder (purity: 99.99% by weight, manufactured by Kojundo Chemical Laboratory Co., Ltd.) at 600 ° C. for 4 hours using an electric furnace. This indium oxide is further pulverized to a fine powder, made into a paste using 1.3-butanediol (dispersion medium), and covered with a platinum wire coil (wire diameter 20 μm) as a noble metal wire, and a spherical shape having a diameter of 0.50 mm. After drying, a current was passed through the platinum wire coil and sintering was performed at 600 ° C. for 1 hour in air to obtain a hot-wire type semiconductor gas detection element. The hot wire type semiconductor gas sensing element is impregnated with an aqueous solution of cerium nitrate, dried and fired, and cerium oxide (Ce ₂ O ₃ ) is added to the indium oxide at various concentrations. Further, a silica thin film was deposited on the hot-wire type semiconductor gas sensing element under the following conditions.

HMDS vapor deposition treatment Processing temperature: 550 ° C Processing time: 25 minutes HMDS vapor pressure: 9 Vol% (35 ° C) Sensitivity measurement voltage: 1.9 V (5.6Ω) (480 ° C)

As described above, gas sensing elements having various added amounts of cerium oxide were prepared, and their sensitivity characteristics were examined. The results are as shown in FIG.

That is, when cerium oxide is added, the rate of increase in relative sensitivity is high even with a high concentration of hydrogen gas of 2000 ppm or more, and high linearity appears.
It can be seen that the concentration can be determined with high accuracy. Also, in this case, it was found that the linearity was still improved even when cerium was added up to about 4 mol%.

Example 2 Similarly, cerium nitrate was used instead of cerium nitrate.
Praseodymium nitrate and terbium chloride
Add praseodymium oxide and terbium oxide to the reaction layer,
The linearity of the hydrogen gas concentration dependence of the relative sensitivity
The results are as shown in FIGS. From the figure
Addition of additives improves the linearity of the graph
You can read it. Further, the lanthanide metal is
Jim, Samarium, Europium, Gadolinium, Dis
Prosium, holmium, erbium, ytterbiu
And lutetium, use the chloride of each metal
6 to 14 and the results were as shown in FIG.
Neodymium (Nd_TwoO_Three), Samarium oxide (Sm_TwoO
_Three), Europium oxide (Eu)_TwoO_Three), Gadolinium oxide
Um (Gd_TwoO_Three), Dysprosium oxide (Dy
_TwoO_Three), Holmium oxide (Ho)_TwoO_Three), Oxidized Elbi
Um (Er)_TwoO_Three), Ytterbium oxide (Yb
_TwoO_Three), Lutetium oxide (Lu)_TwoO_ThreeSame for
It can be seen that the tendency is observed.

Comparative Example 1 Similarly, lanthanum nitrate was used in place of cerium nitrate, and lanthanum oxide (La ₂ O ₃ ) was added to the sensitive layer. As a result, the result was as shown in FIG. From the figure, it can be seen that even with the addition of the additive, lanthanum among the lanthanoid metals has little effect of improving the linearity, and that the effect of improving the linearity is unique to the lanthanide metal.

Example 3 Table 1 shows the dependence of the sensitivity of hydrogen, methanol and ethanol on the amount of additives for each of the gas detecting elements obtained in Examples 1 and 2.

[0024]

[Table 1]

From the table, it can be seen that high hydrogen sensitivity was obtained. Incidentally, in a fuel supply system such as a hydrogen vehicle that uses methanol by converting it to hydrogen, the selectivity of hydrogen to methanol is as high as 2000 ppm.
Since it is desired to be lower than 0 ppm sensitivity, 0.04
It can be seen that an addition amount of about 10 to 10 mol% is particularly preferable. Further, for the same reason as above, it is understood that the addition amount of praseodymium is particularly preferably 0.04 to 2 mol%, and the addition amount of terbium is particularly preferably 0.04 to 0.4 mol%.

Example 4 The durability of each gas sensing element obtained in Example 1 to which 0.04, 0.2, 1.4 mol% of cerium oxide was added, against exposure to high-concentration hydrogen was examined. 16 to 19 in the order of description.
The hydrogen gas exposure conditions were 1% hydrogen gas for 10 minutes, and the degree of gas sensitivity recovery was 30 minutes after the exposure test.
It was examined by gas sensitivity after maintaining the energized state under normal environment for 5 minutes. When performing multiple times, 3
The operation up to the 0 minute energization was performed a plurality of times in the same manner.

As a result, even when a high concentration of hydrogen was exposed at any amount of addition, it was found that there was almost no change in sensitivity, and highly reliable gas detection could be performed.

Example 5 In place of cerium oxide, praseodymium oxide, terbium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, dysprosium oxide, holmium oxide, erbium oxide, ytterbium oxide, and lutetium oxide were used. When the durability of the gas detection element against exposure to high-concentration hydrogen was examined in the same manner as in Example 4, the results were as shown in FIGS. Note that FIG.
In ~ 30, the amount of the additive was 2 mol%. As a result, it can be seen that other lanthanide metals also exhibit the same effect as cerium, and that the same effect can be expected with lanthanide metals.

[Comparative Example 2] A hot-wire type semiconductor gas sensing element was formed in the same manner as above except that the above-mentioned additive was not added to the sensitive layer mainly containing an indium oxide semiconductor. The change in gas sensitivity when exposed to hydrogen gas was investigated. The recovery of gas sensitivity after exposure was also investigated. As a result, the result was as shown in FIG.

That is, it can be seen that the hydrogen sensitivity is greatly reduced after exposure to a high concentration of hydrogen gas. Therefore, it can be seen that such high durability against exposure to high concentration hydrogen is due to the addition of the lanthanide metal oxide.

[Another Embodiment] Another embodiment will be described below.
You. In the previous embodiment, the acid used to make the paste
Of commercially available indium hydroxide to obtain indium chloride
However, the solution was infused from an aqueous solution of indium salt such as indium chloride.
Of indium hydroxide through hydrolysis by ammonia, etc.
Oxidation by washing, drying and baking while obtaining a precipitate
Indium may be obtained. In addition,
In the embodiment, HMDS was used to form a silica thin film.
Is a halosilane (SiX_xH_4-x), Alkyl silane
(R_xSiH_4-x), Alkylhalosilanes (R_xSix
_4-x), Silyl alkoxide (RO)_xSi (OH)_Four-
x(Where X is a halogen, R is an alkyl group, and x is
Is an integer of 1 to 4;
I don't know. ) Can be used.
You. In such a case, it seems that the condition is different from HMDS
However, under conditions similar to those described above, a dense silica thin film
Is expected to be obtained. In the present invention,
Precious metal wire is not limited to platinum wire coil
Alloys and other precious metals may be used.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional perspective view of a hydrogen gas detecting element.

FIG. 2 is a circuit diagram of a gas detection device.

FIG. 3 is a graph showing the hydrogen gas concentration dependence of the sensitivity of a gas detection element having a sensitive layer to which various amounts of cerium oxide are added.

FIG. 4 is a graph showing the hydrogen gas concentration dependence of the sensitivity of a gas detection element having a sensitive layer with various amounts of praseodymium oxide added.

FIG. 5 is a graph showing the hydrogen gas concentration dependence of the sensitivity of a gas detection element having a sensitive layer to which various amounts of terbium oxide are added.

FIG. 6 is a graph showing the hydrogen gas concentration dependency of the sensitivity of a gas detection element having a sensitive layer to which various amounts of neodymium oxide are added.

FIG. 7 is a graph showing the hydrogen gas concentration dependence of the sensitivity of a gas detection element having a sensitive layer with various amounts of added samarium oxide.

FIG. 8 is a graph showing the hydrogen gas concentration dependency of the sensitivity of a gas detection element having a sensitive layer to which various amounts of europium oxide are added.

FIG. 9 is a graph showing the hydrogen gas concentration dependence of the sensitivity of a gas detection element having a sensitive layer to which various amounts of gadolinium oxide are added.

FIG. 10 is a graph showing the hydrogen gas concentration dependency of the sensitivity of a gas detection element having a sensitive layer to which various amounts of dysprosium oxide are added.

FIG. 11 is a graph showing the hydrogen gas concentration dependence of the sensitivity of a gas detection element having a sensitive layer to which various amounts of holmium oxide are added.

FIG. 12 is a graph showing the hydrogen gas concentration dependence of the sensitivity of a gas detection element having a sensitive layer with various amounts of erbium oxide added.

FIG. 13 is a graph showing the hydrogen gas concentration dependence of the sensitivity of a gas detection element having a sensitive layer to which various amounts of ytterbium oxide are added.

FIG. 14 is a graph showing the hydrogen gas concentration dependency of the sensitivity of a gas detection element having a sensitive layer with various amounts of lutetium oxide added.

FIG. 15 is a graph showing the hydrogen gas concentration dependence of the sensitivity of a gas detection element having a sensitive layer to which various amounts of lanthanum oxide are added.

FIG. 16 is a graph showing the durability of a gas detection element having a sensitive layer containing 0.04 mol% of cerium oxide to high-concentration hydrogen gas exposure.

FIG. 17 is a graph showing the durability of high-concentration hydrogen gas exposure to the sensitivity of a gas detection element having a sensitive layer containing 0.2 mol% of cerium oxide.

FIG. 18 is a graph showing the durability of a gas detection element having a sensitive layer containing 1 mol% of cerium oxide to high-concentration hydrogen gas exposure.

FIG. 19 is a graph showing the durability of high-concentration hydrogen gas exposure to sensitivity of a gas detection element having a sensitive layer containing 4 mol% of cerium oxide.

FIG. 20 is a graph showing the high-concentration hydrogen gas exposure durability of a gas detection element having a sensitive layer containing 2 mol% of praseodymium oxide.

FIG. 21 is a graph showing the durability of a gas sensing element having a sensitive layer containing 2 mol% of terbium oxide to high-concentration hydrogen gas exposure.

FIG. 22 is a graph showing the durability of a gas detection element having a sensitive layer containing 2 mol% of neodymium oxide to high-concentration hydrogen gas exposure.

FIG. 23 is a graph showing the durability of a gas detection element having a sensitive layer with an added amount of samarium oxide of 2 mol% to high-concentration hydrogen gas exposure.

FIG. 24 is a graph showing the high-concentration hydrogen gas exposure durability of a gas detection element having a sensitive layer containing 2 mol% of europium oxide.

FIG. 25 is a graph showing the durability of a gas detection element having a sensitive layer containing 2 mol% of gadolinium oxide to exposure to high-concentration hydrogen gas.

FIG. 26 is a graph showing the durability of a gas detection element having a sensitive layer containing 2 mol% of dysprosium oxide to exposure to high-concentration hydrogen gas.

FIG. 27 is a graph showing the durability of a gas sensing element having a sensitive layer containing 2 mol% of holmium oxide to high-concentration hydrogen gas exposure.

FIG. 28 is a graph showing the durability of a gas detection element having a sensitive layer containing 2 mol% of erbium oxide to exposure to high-concentration hydrogen gas.

FIG. 29 is a graph showing the high-concentration hydrogen gas exposure durability of a gas detection element having a sensitive layer containing 2 mol% of ytterbium oxide;

FIG. 30 is a graph showing the durability of a gas detection element having a sensitive layer containing 2 mol% of lutetium oxide to exposure to high-concentration hydrogen gas.

FIG. 31 is a graph showing the durability of a gas detection element having a cerium oxide-free sensitive layer to high-concentration hydrogen gas exposure.

FIG. 32 is a schematic view illustrating an improvement in gas selectivity by a silica thin film.

[Explanation of symbols]

 1 Noble metal wire 2 Sensitive layer 3 Silica thin film

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2G046 AA05 BA02 BA09 BC03 BD03 BE02 BF01 DB05 DD01 EA10 FB02 FE15 FE18

Claims (5)

[Claims] 1. A gas sensing element provided with a sensitive layer formed of a semiconductor containing indium oxide as a main component and covering a noble metal wire, wherein the sensitive layer includes at least one metal selected from lanthanide metals. 0.4 ~
A hydrogen gas detection element to which 10 mol% is added and a dense silica thin film is formed. 2. A gas sensing element provided with a sensitive layer formed of a semiconductor containing indium oxide as a main component and covering a noble metal wire, wherein the sensitive layer includes at least one metal selected from lanthanide metals. 0.4 ~
A hydrogen gas detection element to which 10 mol% is added and a hydrogen selective permeable silica thin film is formed on a sensitive layer. 3. A gas sensing element provided with a sensitive layer formed of a semiconductor containing indium oxide as a main component and covering a noble metal wire, wherein the sensitive layer includes at least one metal selected from lanthanide metals. 0.4 ~
A hydrogen gas detecting element to which 10 mol% is added and a silica thin film is formed on a sensitive layer by chemical vapor deposition. 4. The silica thin film, wherein the sensitive layer is a sintered body of indium oxide obtained by adding 0.4 to 10 mol% of an oxide of at least one metal selected from lanthanide metals to the sensitive layer. However, in hexamethyldisiloxane gas, a current is passed through the noble metal wire,
The hydrogen gas detection element according to any one of claims 1 to 3, wherein the hydrogen gas detection element is formed by chemical vapor deposition at 550 ° C for 25 minutes to 25 hours. 5. The method according to claim 1, wherein the lanthanide metal is cerium, praseodymium, terbium, neodymium, samarium, europium, gadolinium, dysprosium, holmium,
The gas detection element according to claim 1, wherein the gas detection element is at least one metal selected from erbium, ytterbium, and lutetium.